Field of the Invention
The present invention relates to a zoom lens with a built-in extender, which is suitable for a television camera, a video camera, a photograph camera, a television broadcast camera, and a movie camera.
Description of the Related Art
In recent years, a zoom lens having a high zoom ratio and an ability to achieve high optical performance has been demanded for an image pickup apparatus such as a television camera. A built-in extender technique has been known as a technique of covering a high zoom ratio with a single zoom lens. The built-in extender technique is a technique of shifting the focal length range of the zoom lens to a longer focus side by inserting an extender lens unit into a space reserved inside a relay lens unit which is not moved for zooming.
Japanese Patent Application Laid-Open No. 2011-75646 proposes a zoom lens suitable for a ⅔″ television broadcast camera, the zoom lens including four lens units, and having a zoom ratio of 12× and an extender magnification of 2×.
In general, the extender lens unit may be divided into a front lens sub-unit and a rear lens sub-unit with the largest air space interposed therebetween. The front lens sub-unit having a positive refractive power converges substantially afocal input rays to reduce the beam height, and then the rear lens sub-unit having a negative refractive power converts the rays into substantially afocal rays again and outputs the resultant rays.
Using the lens system of Japanese Patent Application Laid-Open No. 2011-75646 as it is with the same lens total length and the same extender magnification, the zoom lens is required to improve its peripheral performance, in particular, in order to achieve further performance enhancement. To attain an extender with a high zoom ratio and high performance entails deterioration of the peripheral performance due to not only increases in aberration amounts but also a difficulty in correcting the Petzval sum. To avoid this, the arrangement of lenses in the extender needs to be set appropriately.